This invention relates to threaded joints and is particularly relates to power threads, such as five thread buttress joints for oil well tubing, casing and the like.
For modern demanding oil field use, pipe coupling systems must satisfy a number of often conflicting requirements. Thus, efficient oil well tubing and casing joints must provide a very good seal against the high pressures encountered both from inside and outside sources (particularly in deep wells), and have structural integrity, as well as good tensile and torsion force absorption characteristics. It is particularly important for reduction of costs in field use that the pipe couplings can be made and broken repeatedly, without galling or other damage to the threads.
Couplings for tubing and casing in critical oil field applications are often based on the "five thread buttress" A.P.I. standard, so-called because it has five threads per inch. These are referred to as power threads because they are more capable of handling high loads than other standard threads, such as A.P.I. round. The buttress thread is characterized by a generally rectangular profile with a stab flank on the side of the pipe that enters the coupling, a load flank on the opposite side which is to absorb the load in tensile, and an approximately flat crest. The standard A.P.I. five thread buttress has a 0.060" height and a taper of 3/4 inch per foot, which is 0.0625" per inch of length. The metal usually employed is that suited for the more demanding applications, typically being of high alloy stainless steel or 9% to 13% chrome alloys with carbon steel. Such materials, however, are particularly subject to galling because of their frictional characteristics. In practice galling can appear on 50% to 90% of the connections in a string where such materials are used. Despite the fact that these tubular products are initially more expensive, therefore, they are disproportionately more expensive because of field service problems.
U.S. Pat. No. 3,109,672 to Franz is representative of prior art patents relating to buttress type couplings. This patent points up the problems of galling, poor repeated make-up characteristics of tubing joints and the hoop stress problems in the coupling. The design of the tapered buttress joint described and shown in the Franz patent is particularly characterized by low hoop stress in the end of the coupling. Other examples of prior art threaded tube joints employing buttress threads are given in U.S. Pat. Nos. 3,224,799; 2,062,407; 2,177,100; 2,006,520; 2,772,102 and 3,989,284. However, in most cases these joints have not been found practical for use in modern oil field technology for various reasons, such as being readily subject to damage, excessive galling, leakage and high cost. The buttress thread is widely used but as noted practical difficulties remain in a number of respects. There is still a need therefore for a tubing and casing joint particularly suited for demanding oil well applications which has high tensile strength, and excellent sealing characteristics, which is essentially free from galling during repeated make-up and break-out operations, and which imposes no added cost because of design complexities.